Selective erasure of a bistable storage tube

ABSTRACT

A bistable cathode-ray storage tube including a phosphor storage layer with a collector electrode in contact with the image storing side thereof is selectively erased with the same electron beam employed for writing stored images. Emission from the tube&#39;&#39;s flood guns is discontinued, while the relative voltage differential between the collector electrode and a target electrode underneath the phosphor layer is altered for increasing the voltage difference between written areas of the charge image and the potential of the collector electrode. The writing beam is then deflected in a raster configuration for erasing a selected portion of a stored image by charging down such selected portion.

United States Patent lnventor Kent H. Johnston Beaverton, Oreg.

Appl. No. 885,752

Filed Dec. 17, 1969 Patented Oct. 5, 1971 Assignee Tektronix, Inc.

Beaverton, Oreg.

SELECTIVE ERASU RE OF A BISTABLE STORAGE TUBE References Cited UNITED STATES PATENTS 3,331,983 7/1967 Hesse 315/12 3,368,093 2/1968 Sjoberg 313/68 Primary Examiner-Rodney D. Bennett, Jr. Assistant Examiner-N. Moskowitz Attorney-Buckhorn, Blore, Klarquist and Sparkman ABSTRACT: A bistable cathode-ray storage tube including a phosphor storage layer with a collector electrode in contact with the image storing side thereof is selectively erased with the same electron beam employed for writing stored images. Emission from the tube's flood guns is discontinued, while the relative voltage differential between the collector electrode 15 claimss Drawing Figs and a target electrode underneath the phosphor layer is al- U.S.Cl 315/12, tered for increasing the voltage difference between written 313/68 areas of the charge image and the potential of the collector Int. Cl 1101] 29/41 electrode. The writing beam is then deflected in a raster con- Field of Search 315/11, 12, figuration for erasing a selected portion of a stored image by 13; 313/68 charging down such selected portion. I-3o7ov -3 050V.

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v. TARGET ELEcTRooE VOLTAGE 325 WRITING GUN BEAM OFF +47ov mu- 4 WRITTEN PI-IOsPI-IOR SURFACE POTENTIAL +|V. \lfw +325v BACKGROUND PHOSPHOR J +295v. SURFACE POTENTIAL L ov.

TINE g TARGET ELECTRODE 0v VOLTAGE 1: ISOV- HSOVI COLLECTOR v VOLTAGE FIG 5 WRITING GUN BEAM l I OFF v WRITTEN PHOSPHOR 'x URFACE POTENTIAL 0v ov. BACKGROUND PHOSPHOR suRFAcE POTENTIAL TIME KENT H. JOHNSTON INVENTOR BUCKHORN, BLORE, KLARQUIST & SPARKMAN ATTORNEYS 1 SELECTIVE ERASURE OF A BISTABLE STORAGE TUBE CROSSREFERENCE TO RELATED APPLICATION This application is related to the application of Roger A. Frankland, Ser. No. 816,576, filed Apr. 16, 1969, entitled Bistable Storage Tube Having Fast Erase Speed.

BACKGROUND OF THE INVENTION In the aforementioned application of Roger A. Frankland, a bistable storage tube is described and claimed having a phosphor storage layer disposed over a target electrode, and further having a collector electrode in contact with the image storing side of the phosphor layer. The tube is provided with a writing electron gun for writing stored information upon the phosphor layer, and flood guns for directing low-velocity electrons at the phosphor layer. When electrons from the writing gun strike an elemental area of the phosphor layer secondary emission of electrons causes such elemental area to become relatively positive. The potential of such elemental area exceeds the first crossover voltage on the secondary emission characteristic curve of the phosphor layer, and therefore electrons from the flood guns will be attracted with sufficient velocity for retaining such area in a positive state.

Generally, in tubes of this type, erasure of the entire phosphor layer is accomplished at the same time, or at least, specified areas having a common target electrode are erased at the same time. Thus, there is no provision for erasing a selected portion of the stored image, without also erasing other information which one may wish to retain.

SUMMARY OF THE INVENTION According to the present invention, a bistable storage tube of the aforementioned type is selectively erased, that is, a specific area anywhere on the phosphor storage layer may be erased with retention of any or all of the rest of the stored data. According to the method of the present invention, the flood guns are disabled and the voltage difference on the target and collector electrodes is altered for increasing the voltage difference between written areas of a charge image and then potential of the collector electrode. A selected area is then bombarded with the writing guns electron beam. The beam is deflected in a systematic pattern, e.g. in a raster pattern, which may be enlarged, contracted, or positioned anywhere upon the phosphor layer. Electron bombardment caused both written and nonwritten areas of the phosphor storage layer, within the raster area, to fall in potential toward the potential of the collector electrode. After the area desired to be erased has charged down below the first crossover voltage of the secondary electron emission curve for the phosphor layer, normal collector potentials and emission from the tube '5 flood guns are restored. The bombarded area then changes to flood gun potential as a result of the action of low-velocity electrons from the flood guns.

It is an object of the present invention to provide an improved method for erasing any area of a bistable storage tube at the selection of the operator.

It is another object of the present invention to provide an improved method for selectively erasing a bistable storage tube, with good resolution of the erased area.

It is a further object of the present invention to provide an improved method for selectively erasing a bistable storage tube in a minimum time without disturbing other stored data which is not to be erased.

The subject matter which I regard as my invention is particularly pointed out and distinctly claimed in the concluding portion of this specification. The invention, however, both as to organization and method of operation, together with further advantages and objects thereof, may best be understood by reference to the following description taken in connection with the accompanying drawings wherein like reference characters refer to like elements.

DRAWINGS FIG. I is a schematic view of a storage apparatus employing a bistable storage tube;

FIG. 2 is a sectional view through thestorage target taken at 22 in FIG. 1;

FIG. 3 is a plot of target secondary emission ratio versus potential on the bombarded side of the phosphorstorage layer for the bistable storage tube in accordance with the present invention;

FIG. 4 is a first chart of waveforms illustrating a first selective erase method according to the present invention; and

FIG. 5 is a chart of waveforms illustrating a second selective erase method in accordance with: the present invention.

DETAILED DESCRIPTION As illustrated in FIG. 1, a storage tube operated according to the present invention is of the direct viewing bistable type set forth in the copending application of Roger A. Frankland, Ser. No. 816,576, filed Apr. 16, 1969, entitled BISTABLE STORAGE TUBE HAVING FAST ERASE SPEED." The tube includes a storage target 12 supported on a light transparent glass faceplate 14 forming part of the evacuated tube envelope hereinafter described in greater detail with respect to FIG. 2. The storage tube includes a conventional writing gun, the cathode of which may be connected to a negative DC supply voltage of about 3,000 volts. The writing gun also includes a control grid 18, a focusing and accelerating anode structure 20, horizontal deflection plates 22, and vertical deflection plates 24. Control grid l8'is connected to switch 72 for selecting the proper grid bias. For normal writing of a stored image, the switch 72 is in the left-hand position, as shown, e.g. selecting a negative 3,050 volts. When the tube is used in a cathode-ray oscilloscope, the input signal is applied to the vertical deflection plates 24 through a vertical amplifier 26, while a ramp voltage sweep signal is applied to the horizontal deflection plates 22 by sweep generator 28. The sweep generator may be triggered in response to receipt of the vertical input signal at input terminal 30 by transmitting a portion of such input signal to trigger generator 32, the output of which is connected to the sweep generator.

In addition, in accordance with the present invention, a raster generator is selectively connected to the horizontal deflection plates 22 and the vertical deflection plates 24 by means of operating switches 82 and 84 to their left-hand position from the normal position illustrated in FIG. I. A positioning potentiometer 86, connected to a positive or negative voltage at its respective ends, has its movable tap connected to the raster generator output for the horizontal deflection plates, while a similar potentiometer 88 has its movable tap connected to the raster generator output applicable to the vertical deflection plates. Raster generator 80 suitably provides conventional television-type raster signals for the horizontal and vertical deflection plates, e.g. for causing deflection of electron beam 74 to produce a plurality of closely spaced horizontal traces. Potentiometers 86 and 88 are utilized for positioning the resulting raster upon target 12. The amplitude of the output of raster generator 80 is suitably controlled for enlarging or diminishing the size of the raster as desired. Raster tracing may be used for erasure as hereinafter described.

A charge image is written on the storage dielectric phosphor layer of the target 12 by the high-velocity electron beam 74 of the writing gun. A positive charge is written by high-energy electron beam 74, with secondary emission of electrons exceeding primary emission of the beam whereby an area of phosphor upon which the beam is directed is written positive. The potential of such area exceeds the first crossover voltage on the secondary electron emission characteristic curve of such phosphor layer, and the charge image may be stored bistably through uniform bombardment of the target with flood electrons. The flood electrons are emitted from a pair of flood guns having cathodes 34, control grids 35, and focusing anodes 36. The flood gun cathodes 34 are grounded while the focusing anodes are connected to a positive 100 volts. The grids 35 are coupled by way of switch 76 to either 25 volts or l volts. For normal writing of a stored image, the switch 76 is in the position shown for selecting the negative 25 volts. A plurality of suitable collimating electrodes may be provided to cause the flood electrons to strike the storage dielectric at right angles thereto. One such collimating electrode 38 is shown as a wall band coated on the inner surface of the tube envelope and connected to a source of positive DC supply voltage of approximately +50 volts.

As illustrated in FIG. 2, the storage target 12 includes a storage dielectric which, for the direct viewing bistable storage tube, advantageously comprises a continuous or undivided layer 40 of phosphor material such as manganese activated zinc orthosilicate, referred to as Pl phosphor, coated on the inner surface of faceplate 14. Target electrode 42 is provided beneath the phosphor layer 40 in the form of a light transparent film of conductive material such as tin oxide, coated on the inner surface of the faceplate 14. The faceplate 14 is a flat plate of transparent glass which serves as the support member for the storage target. An electron permeable collector electrode 44 is positioned in contact with the bombarded surface of phosphor layer 40 on the opposite side of such layer from the target electrode 42. The collector electrode 44 advantageously comprises a mesh electrode of light reflecting material such as aluminum, coated on the phosphor layer by vapor deposition through a suitable mask. The resulting collector mesh electrode is extremely thin and has a fine mesh which may be 200 lines per inch with an electron trans mission of 80 percent. Thus, in this 80-percent transmission embodiment, the phosphor is only 20 percent aluminized by the collector electrode.

The storage tube envelope includes a funnel portion 58 of crystalline ceramic material such as Fosterite, which is sealed in the glass faceplate 14 by an intermediate seal portion 60 of crystallized glass material or a glass different from that of the faceplate, as disclosed in US. Pat. No. 3,207,936 of W. H. Wilbanks, et al., issued Sept. 21, 1965. Tin oxide coatings, forming the collector lead 46 for the collector electrode 44 and the lead portion 47 for target electrode 42, extend through the seal between the seal portion 60 and the faceplate 14, out to the exterior of the tube envelope where they are respectively connected. The target electrode 42 is connected to a switch 90 for selecting between zero volts and +325 volts, the former being chosen for writing operation of the storage tube. Collector lead 46 is insulated from the target electrode and applies an operating voltage to collector electrode 44 from switch 78. Switch 78 selects either positive 150 volts or negative 150 volts, with the former being chosen for normal storage writing operation. This voltage enables the collector electrode 44 to collect secondary electrons emitted from the storage dielectric layer 40 due to bombardment by primary electrons emitted from either the writing gun cathode 16 or the flood gun cathodes 34, since such collector electrode is positive with respect to such cathodes.

Considering storage operation in greater detail, a written area is retained at a relatively positive potential, e.g. corresponding nearly to that of collector electrode 44, after beam 74 has passed a given written area because of the action of the flood guns. The flood guns produce relatively low-velocity electrons which strike the target but which ordinarily have insufficient velocity for writing information. When the electrons from the flood guns strike areas of the target upon which a positive charge has not been written, these flood electrons tend to maintain such areas at the relatively negative potential of the flood guns, e.g. at ground level or zero volts. This is the stable negative potential level of the target. However, in other areas, a positive charge image may be written by electron beam 74 with secondary emission exceeding primary emission whereby a given area is written positive. The secondary electrons are collected by collector electrode 44. The flood gun electrons are attracted to the positive written areas and obtain a high velocity with respect to these areas for producing continued secondary emission therefrom. Therefore, these areas are maintained relatively positive, near the potential of collector electrode 44. This comprises the stable positive potential level of the target. The target thus has bistable properties and is capable of retaining information written thereon, with the flood beam of electrons driving target areas toward one of two stable potentials depending upon information written thereon 0 with beam 74.

The manner in which storage operation takes place will be further described with reference to FIG. 3, a plot of secondary emission versus target potential for the side of the target and specifically the phosphor layer 40 which is bombarded by electron beam 74. Examining the curve of FIG. 3, we see two points at which the secondary emission ratio for the target is equal to one. At V 8=l because the target, and specifically the beam side of the phosphor layer, has collected sufficient electrons to charge a few tenths ofa volt negative with respect to the flood gun cathode, thereby rejecting all electrons. At V the accelerating potential is high enough for the material on the phosphor surface to emit secondary electrons, and at V,, the phosphor layer has charged a few volts higher than the collector electrode, and all secondary electrons in excess of primary electrons are returned to the target. V,, and V, are the stable potentials. If the phosphor layer begins to rise above V,,, the layer collects electrons, the secondary emission being less than one, and the phosphor layer charges negatively, restoring the phosphor layer to V,,. If we bombard the target with a high-energy electron beam 74, and allow the phosphor layer to charge by secondary emission to any potential just under V it will return under the action of the flood guns to V However, if we allow it to charge more positive than V,., due to the action of beam 74, the secondary emission caused by the flood electrons will discharge the phosphor layer positively until it reaches V lfit passes V the secondary emission ratio becomes less than one, and any electrons arriving attempt to charge the phosphor layer negatively. V, is described as the first crossover voltage of the secondary emission characteristic or the minimum voltage necessary for storage.

The voltage level at V, is also the writing threshold level, above which electron beam 74 must bring an elemental area of the phosphor layer in order for the flood beams to take over and retain such elemental area at a stable positive potential nearly equal to the potential of electrode 44. This is the positive stable potential level of the target. All areas which have not been raised above this writing threshold by electron beam 74 will be retained by the flood beam electrons at a voltage close to the potential of the flood gun cathode, i.e. zero volts or the stable negative potential level for the target. Charging of the phosphor layer is understood to mean charging of the exposed surface thereof relative to target electrode 42, with the phosphor layer forming the dielectric of a capacitor.

According to the present invention, an area of the target is selectively erased without changing the information stored on other areas of the target. Two method embodiments of accomplishing this erasure are described and claimed herein. In accordance with each of these methods, the relative voltage differential on electrodes 42 and 44 is altered for increasing the voltage difference between written areas, i.e. relatively positive areas of the target, and the potential of the collector electrode. The increased voltage difference between the written areas of the charge image and the potential of the collector electrode is greater than the initial difference between written and unwritten areas of the phosphor layer. This voltage difference is arranged so that the written areas are momentarily relatively positive as compared to the potential of collector electrode 44. Also, the flood beams from the flood guns are momentarily discontinued, while an area to be selectively erased is bombarded with electrons from the writing gun, i.e. by electron beam 74. With electron bombardment, written areas are charged down toward the potential of the collector electrode. Bombarded areas are ordinarily driven toward collector potential, and writing gun electrons are collected at the surface of the phosphor layer even though the secondary emission ratio is greater than one. The normal voltage differential difference between target and the collector electrodes is then restored, together with restoration of operation of the flood guns, such that normal writing operation can be continued. The period during which the phosphor layer is bombarded with high-velocity electrons from the writing gun is long enough to lower such selected area to a voltage level which will be below the first crossover voltage of thetsecondary emission characteristic of the phosphor layer after' the normal voltage differential is restored. Therefore, flood electrons now impinging upon the erased portion of the phosphor layer will drive it toward the nonwritten stable potential of the target, and these areas will be retained in the nonwritten state due to the normal action of the flood beams.

Referring to a first specific method according to the present invention, the waveform chart of FIG. 4 will be considered. It is assumed that certain areas of the phosphor layer 40 have been written to provide a relatively positive charge image near the potential of the collector electrode 44, while nonwritten areas reside at the potential of the flood gun cathodes, i.e. zero volts. It will be assumed also that the writing gun is now off, with the switch 72 connecting grid 18 to a 3,l50 volts. Otherwise, at this time, the position of various switches in FIGS. 1 and 2 will be as shown. In order to produce erasure of a specific area, the flood guns are first shut off as indicated in FIG. 4. This can be accomplished by operating switch 76 so that l00 volts is provided at flood gun grids 35 instead of -25 volts. The target electrode voltage is now switched from zero volts to +325 volts through operation of switch 90 to its upper position. Both written areas and background areas on the exposed surface of phosphor layer 40 will rise by 325 volts as a consequence of capacitive coupling from target electrode 42. The change in target electrode voltage for erasure is greater than the normal voltage difference (about 150 volts) between written and unwritten areas of the phosphor layer. Also, the target electrode is raised in this instance to a poten tial higher than that of the collector electrode 44. Switch 72 is now moved to its left-hand position applying a 3,050 volts to the writing gun, and the writing gun is deflected to a location where erasure is desired. As can be seen in the lower two waveforms in FIG. 4, both the written and background nonwritten areas of the phosphor layer charge downward toward the ISO-volt collector voltage.

The time required to bring about erasure is the time necessary for charging the elemental area of the target to be erased down to a voltage level which is reducible below the first crossover of the secondary emission characteristic of the phosphor layer, by restoration of normal potential conditions. Thus, after a predetermined period of time, the writing gun is deactivated by returning switch 72 to its right-hand position, whereby the area to be erased received no more negative charge. Now, the target electrode voltageis returned to zero volts by returning switch 90 to its lower position. Through capacitive coupling, written and background areas of the phosphor layer are lowered by 325 volts to 60 volts and -30 volts respectively. In the case of a particular constructed embodiment, +60 volts was below the first crossover voltage" of the secondary emission characteristic for the phosphor layer. Now the flood guns are once more turned on, and the lowvelocity electrons thereof charge former written areas negatively toward zero volts and background areas positively toward zero volts, completing erasure of a particular target area. The specific time for erasure of an elemental area of the target depends upon the current in the beam because the total charge delivered per unit area is a determining factor for erasure. In a typical example, the time required for erasing an elemental area corresponding to the trace of the stationary electron beam was about microsecond.

Rather than just erasing an elemental area, it is generally desired to erase some particular larger portion of the stored image. For this purpose, a raster generator 80 is employed for causing scanning of electron beam 74 in a systematic manner over a desired area of the stored image. Switches 82 and 84 are in their left-hand position so that the raster outputs are applied to the horizontal and vertical deflection plates of the tube. The raster generator is suitably controlled to adjust the amplitude of the outputs thereof so that a raster having a size corresponding to the size of the erased area is generated. The movable taps on potentiometers'86 and 88 are employed for positioning of such raster upon the phosphor layer. Thus, the raster is generally small compared with the overall size of phosphor layer 40, and may be positioned to a particular portion of the image which it is desired to erase.

The tube illustrated and described inthe aforementioned Frankland application is suitable of the direct view type whereby the stored information is observed through glass faceplate 14. When a particular area is to be erased, switch 72 is first thrown to the middle position, e.g, selecting a 3,070 volts, and the raster generator is energized with" switches 82 and 84 connecting the raster generator to the deflection plates. The bias forwriting gun grid 18 selected by switch 72 in its middle position provides an electron beam 74 which has insufficient average current density for writing of stored information upon the phosphor layer 40. That is, with switch 72 connected as described, electron beam 74 will notraise a portion of the target above the first crossover voltage of the secondaryemission characteristic ofrthe phosphor layer, during raster scanning. However, the raster so generated may be viewed through faceplate 14 for positioning such raster upon the phosphor layer. Thus, potentiometers 86 and 88 may be adjusted for moving the raster in the X and Y directions, respectively, while-controls of the raster generator may be employed to magnify or de'magnify the size of the raster itself by varying the output amplitudes. Then, after such raster is positioned over an area of trace information which it is desired to erase, the hereinbefore described erasing procedure is suitably followed.

The raster generator suitably generates deflection signals for producing a series of closely spaced horizontal traces across phosphor layer 40 in a horizontal direction. That is, a first sawtooth voltage is applied between vertical deflection plates 24, and a faster sawtooth voltage is applied between horizontal deflection plates 22 for generating a raster in the usual manner as understood by those skilled in the art. However, the speed of the deflection in the case of erasure should not be so rapid, nor the size of the raster used for erasing so large, that insufficient charge is deposited in the erase cycle to produce the erasing effect. in general, about 1 microsecond is required forerasing a lO-mil diameter circle on the face of the cathode-ray tube. Expressed in another manner, the erase factor, equaling the number of electrons required, is approximately 6X10 electrons per 4 square mil area. These figures are given by way of example for the particular apparatus employed.

It will also be understood that the switching as accomplished by switching" devices 72, 76, 82, 84, and 90 is more suitably accomplished with electronic switching apparatus than with actual manually operated switches. Although a raster trace is suitably employed for erasure, the speed of erasure is quite rapid and comparable with that of the aforementioned Frankland application since erasure is limited only by the amount of beam current available during erase time. The period for total erasure is sufficiently short so that the stored image is not lost during the erasure period although flood electrons are temporarily absent.

As hereinbefore mentioned, the change in target electrode voltage for erasure is greater than the voltage difference between written and unwritten portions of the phosphor layer or the normal voltage difference between the jcollector electrode 44 and the vflood gun cathodes. Thus,,the target electrode pulse for erasure in the present example was 325 volts in amplitude, while the difference between the flood gun cathode voltage and the collector voltage was I50 volts. in general, the change in the target electrode voltage should be at least approximately. twice the difference in voltage between written and unwritten areas on the phosphor layer. Thus, since the target electrode voltage is initially zero volts, the target electrode is raised from a voltage level below that of the collector electrode to a voltage level considerably above that of the collector electrode. This voltage pulse is capacitively coupled to the electron beam side of the phosphor layer, and establishes the voltage levels at which both written and unwritten areas start to charge down toward the collector electrode voltage. Therefore, if the target electrode voltage is high, the written and unwritten areas of the phosphor storage layer will charge down more rapidly. Thus, employing the higher voltage pulses speeds the erase operation.

Moreover, the high-voltage target electrode pulse, of at least twice the written and nonwritten differential, enhances the resolution of erasure and maintains trace edge integrity. When the writing gun is employed for erasure as hereinbefore described, secondary electrons are produced which themselves tend to land on other portions of the phosphor layer near the area which is being erased. With the higher target electrode pulse, this efiect is minimized, since the primary beam and secondary electrons are confined by the higher voltage. It is noted the same problem of beam integrity does not occur during writing since the secondary electrons themselves do not have sufficient energy for writing and are ineffective for erasure in the presence of the flood beam. However, when the writing beam is used for erasure, the secondary electrons themselves may produce erasure, unless the beam is confined by a high-voltage pulse.

FIG. illustrates a second method of selective erasure according to the present invention. According to this method, the collector electrode voltage is rapidly reduced from a +150 volts to a l50 volts, after which the writing gun beam is turned on and directed to a particular area where erasure is desired. The change in collector electrode voltage from +150 volts to l 50 volts is accomplished through operation of switch 78 in H6. 2. However, switch 78 is merely illustrative of a more rapid voltage changing means, since the change in collector voltage should be very rapid. In this case, not only is the collector voltage change employed for providing a low voltage toward which erased portions of the phosphor layer will charge, but also the collector voltage pulse is employed in effect to turn off the flood beams. Thus, the negative 150 volts on electrode 44 is effective for repelling the low-velocity electrons generated by the tube s flood guns. The transition from +150 volts to l50 volts and the reverse change at the end of the pulse must be short enough so that surrounding stored areas will not have time to collect a significant charge as less than first crossover energy before the flood beams are cut off. Otherwise, the flood beams would tend to erase the entire phosphor layer. Alternatively, of course, the flood beams may be shut off first through operation of switch 76. It is noted that the collector electrode is lowered from a voltage positive relative to the voltage level of the flood gun cathodes and the target voltage to a voltage which is negative relative to the voltage level of the flood gun cathodes and target electrode.

After the collector electrode 44 has been lowered in potential, the writing gun is turned on by operating switch 72 to its left-hand position. The resulting electron beam will drive areas to which it is directed toward the lower collector electrode voltage. Although the beam has sufficient energy to produce secondary electrons, these electrons, in addition to primary electrons, are retained on the storage surface of the phosphor layer inasmuch as the collector electrode 44 is lower in potential than the surface of the phosphor layer. Thus, written areas are driven from +145 volts, negatively, while background or unwritten areas are also driven negatively, as a result of electrons accumulating in these areas. The period of time required to erase a given area is the time required for lowering that area to a level which will be below the first crossovcr voltage in the secondary emission characteristic for the phosphor layer under normal electrode voltage conditions. Here, the written areas are lowered to a +60 volts while unwritten areas are lowered to a -85 volts. Then, the writing gun is turned off, and the collector voltage is again raised to +150 volts. As a result of the action of the flood electrons, which are no longer repelled by the collector voltage, both written and nonwritten areas covered by the writing gun beam during erasure will be returned to zero volts. The erasure time for an elemental area is again about 1 microsecond. Also, the raster generator may be employed as hereinbefore described for selectively erasing a particular area which can be predetermined, i.e. by first writing the raster in a nonstoring mode as hereinbefore described. As in the previously described method embodiment, the change in voltage employed for producing a voltage difference between the written areas of the charge image and the collector electrode voltage is desirably at least approximately twice the voltage between stored and nonstored areas. Again, the advantages ensuing therefrom are speed of operation, and confining of erasure to the desired selected area.

While I have shown and described preferred embodiments of my invention, it will be apparent to those skilled in the art that many changes and modifications may be made without departing from my invention in its broader aspects. 1 therefore intend the appended claims to cover all such changes and modifications as fall within the true spirit and scope of my invention.

lclaim:

l. The method of operating a bistable storage tube, said tube including a phosphor layer on an insulative support member, a target electrode on said support member beneath said phosphor layer, a collector electrode supported in contact with said phosphor layer on the opposite side of said phosphor layer from said target electrode for collecting secondary electrons, a writing electron gun for providing a beam of high-velocity electrons, and holding means for bombarding the phosphor layer with low-velocity electrons, said method comprising:

writing a charge image on selected areas of said phosphor layer by bombarding said phosphor layer with said beam of high-velocity electrons from said writing gun in said selected areas, with said low-velocity electrons from said holding means producing continued secondary emission from said selected areas to cause bistable storage of such image,

discontinuing emission of said low-velocity electrons from said holding means,

altering the relative voltage differential on said target and collector electrodes for increasing the voltage difference between the written areas of said charge image and the potential of said collector electrode, with the collector electrode being relatively negative compared with written areas of said phosphor layer, said difference being greater than the voltage difference between written and unwritten areas of said phosphor layer,

and, while said relative voltage differential is altered, bombarding a selected area of said phosphor layer to be erased with said beam of high-velocity electrons from said writing gun for erasing information from a portion of said phosphor layer toward which said beam of high-velocity electrons is directed by charging written areas downward toward the potential of the collector electrode.

2. The method according to claim 1 wherein a selected area of said phosphor layer to be erased is bombarded with said beam of high-velocity electrons for a period long enough to lower such selected area to a level which resides below the first crossover voltage of the secondary electron emission characteristic of said phosphor layer after subsequent restoration of said relative voltage differential.

3. The method of operating a bistable storage tube, said tube including a phosphor layer on an insulative support member, a target electrode on said support member beneath said phosphor layer, a collector electrode supported in contact with said phosphor layer on the opposite side of said phosphor layer from said target electrode for collecting secondary electrons, a writing electron gun for providing a beam of high-velocity electrons, and holding means for bombarding the phosphor layer with low-velocity electrons, said method comprising:

writing a charge image on selected areas of said phosphor layer by bombarding said phosphor layer with said beam of high-velocity electrons from said writing gun in said selected areas, with said low-velocity electrons from said holding means producing continued secondary emission from said selected areas to cause bistable storage of such image,

discontinuing emission of said low-velocity electrons from said holding means,

and raising the potential on said target electrode by more than the voltage difi'erence between written and unwritten areas of said phosphor layer,

and while said target electrode is thus raised in voltage level and said low-velocity electrons are discontinued, operating said writing gun for bombarding said phosphor layer with said beam of electrons for erasing information from a portion of said phosphor layer toward which said beam of high-velocity electrons is directed.

4. The method according to claim 3 wherein the electron beam from said writing gun during erasure is systematically deflected in a predetermined pattern over an area wherein erasure is desired.

5. The method according to claim 4 wherein said deflection of said electron beam from said writing gun for erasure comprises deflecting said writing beam in a raster sequence over a selected area to be erased.

6. The method according to claim 3 wherein said potential on said target electrode is raised from a voltage level below that of said collector electrode to a voltage level above that of said collector electrode.

7. The method of operating a bistable storage tube, said tube including a phosphor layer on an insulative support member, a target electrode on said support member beneath said phosphor layer, a collector electrode supported in contact with said phosphor layer on the opposite side of said phosphor layer from said target electrode for collecting secondary electrons, a writing electron gun for providing a beam of high-velocity electrons, and holding means for bombarding the phosphor layer with low-velocity electrons, said method comprising:

writing a charge image on selected areas of said phosphor layer by bombarding said phosphor layer with said beam of high-velocity electrons from said writing gun in said selected areas, with said low-velocity electrons from said holding means producing continued secondary emission from said selected areas to cause bistable storage of such image,

lowering the voltage of said collector electrode from a positive voltage relative to the voltage level of said holding means to a negative voltage relative to the voltage level of said holding means,

and, while the collector electrode voltage is thus lowered,

operating said writing gun for bombarding said phosphor layer with said beam of electrons for erasing information from a portion of said phosphor layer toward which said beam of high-velocity electrons is directed.

8. The method according to claim 7 wherein said voltage on said collector electrode is lowered by more than the voltage difference between written and unwritten areas of said phosphor layer.

9. The method according to claim 7 wherein the electron beam from said writing gun during erasure is deflected over an electrode voltage is rapidly reduced for effectively cutting off operation of said flood guns without erasure of said charge image by said flood guns.

11. The method of operating a bistable storage tube, said tube including a hosphor layer on an insulative support member, a target e ectrode on said support member beneath said phosphor layer, a collector electrode supported in contact with said phosphor layer on the opposite side of said phosphor layer from said target electrode for collecting secon dary electrons, a writing electron gun for providing a beam of high-velocity electrons, and holding means for bombarding the phosphor layer with low-velocity electrons, said method comprising:

writing a charge image on selected areas of said phosphor layer by bombarding said phosphor layer with said beam of high-velocity electrons from said writing gun in said selected areas, with said lowvelocity electrons from said holding means producing continued secondary emission from said selected areas to cause bistable storage of such image,

discontinuing emission of said-low-velocity electrons from said holding means,

altering the normal relative voltage differential on said target and collector electrodes for increasing the voltage difference between the written areas of said charge image and the potential of said collector electrode, with the collector electrode being relatively negative compared with written areas of said phosphor layer, said difference being greater than the voltage difference between written and unwritten areas of said phosphor layer,

bombarding a selected area of said phosphor layer to be erased with said beam of high-velocity electrons from said writing gun while deflecting said beam of electrons in a systematic pattern to charge written areas, toward which said beam is directed, downward toward the potential of said collector electrode,

and restoring the normal voltage differential between said target and collector electrodes as well as restoring operation of said holding means for producing secondary electron emission from written areas not erased for retention of information represented thereby.

12. The method according to claim 11 wherein said selected area of said phosphor layer to be erased is bombarded with said beam of high-velocity electrons for a period long enough to lower such selected area to a level which resides below the first crossover voltage of the secondary electron emission characteristic of said phosphor layer after said normal voltage differential is restored.

13. The method according to claim ll including deflecting said beam of electrons in said systematic pattern with a reduced beam current prior to bombarding said selected area of said phosphor layer to be erased,

and adjusting the size and position of said systematic pattern while viewing the same upon said phosphor layer prior to erasure for selecting the area to be erased.

14. The method according to claim 13 wherein said systematic pattern comprises a raster sequence.

15. The method according to claim 11 wherein the increased voltage difference between written areas of the target and the potential of the collector electrode is at least approximately twice the voltage difference between written and unwritten areas of said phosphor layer. 

1. The method of operating a bistable storage tube, said tube including a phosphor layer on an insulative support member, a target electrode on said support member beneath said phosphor layer, a collector electrode supported in contact with said phosphor layer on the opposite side of said phosphor layer from said target electrode for collecting secondary electrons, a writing electron gun for providing a beam of high-velocity electrons, and holding means for bombarding the phosphor layer with low-velocity electrons, said method comprising: writing a charge image on selected areas of said phosphor layer by bombarding said phosphor layer with said beam of highvelocity electrons from said writing gun in said selected areas, with said low-velocity electrons from said holding means producing continued secondary emission from said selected areas to cause bistable storage of such image, discontinuing emission of said low-velocity electrons from said holding means, altering the relative voltage differential on said target and collector electrodes for increasing the voltage difference between the written areas of said charge image and the potential of said collector electrode, with the collector electrode being relatively negative compared with written areas of said phosphor layer, said difference being greater than the voltage difference between written and unwritten areas of said phosphor layer, and, while said relative voltage differential is altered, bombarding a selected area of said phosphor layer to be erased with said beam of high-velocity electrons from said writing gun for erasing information from a portion of said phosphor layer toward which said beam of high-velocity electrons is directed by charging written areas downward toward the potential of the collector electrode.
 2. The method according to claim 1 wherein a selected area of said phosphor layer to be erased is bombarded with said beam of high-velocity electrons for a period long enough to lower such selected area to a level which resides below the first crossover voltage of the secondary electron emission characteristic of said phosphor layer after subsequent restoration of said relative voltage differential.
 3. The method of operating a bistable storage tube, said tube including a phosphor layer on an insulative support member, a target electrode on said support member beneath said phosphor layer, a collector electrode supported in contact with said phosphor layer on the opposite side of said phosphor layer from said target electrode for collecting secondary electrons, a writing electron gun for providing a beam of high-velocity electrons, and holding means for bombarding the phosphor layer with low-velocity electrons, said method comprising: writing a charge image on selected areas of said phosphor layer by bombarding said phosphor layer with said beam of high-velocity electrons from said writing gun in said selected areas, with said low-velocity electrons from said holDing means producing continued secondary emission from said selected areas to cause bistable storage of such image, discontinuing emission of said low-velocity electrons from said holding means, and raising the potential on said target electrode by more than the voltage difference between written and unwritten areas of said phosphor layer, and while said target electrode is thus raised in voltage level and said low-velocity electrons are discontinued, operating said writing gun for bombarding said phosphor layer with said beam of electrons for erasing information from a portion of said phosphor layer toward which said beam of high-velocity electrons is directed.
 4. The method according to claim 3 wherein the electron beam from said writing gun during erasure is systematically deflected in a predetermined pattern over an area wherein erasure is desired.
 5. The method according to claim 4 wherein said deflection of said electron beam from said writing gun for erasure comprises deflecting said writing beam in a raster sequence over a selected area to be erased.
 6. The method according to claim 3 wherein said potential on said target electrode is raised from a voltage level below that of said collector electrode to a voltage level above that of said collector electrode.
 7. The method of operating a bistable storage tube, said tube including a phosphor layer on an insulative support member, a target electrode on said support member beneath said phosphor layer, a collector electrode supported in contact with said phosphor layer on the opposite side of said phosphor layer from said target electrode for collecting secondary electrons, a writing electron gun for providing a beam of high-velocity electrons, and holding means for bombarding the phosphor layer with low-velocity electrons, said method comprising: writing a charge image on selected areas of said phosphor layer by bombarding said phosphor layer with said beam of high-velocity electrons from said writing gun in said selected areas, with said low-velocity electrons from said holding means producing continued secondary emission from said selected areas to cause bistable storage of such image, lowering the voltage of said collector electrode from a positive voltage relative to the voltage level of said holding means to a negative voltage relative to the voltage level of said holding means, and, while the collector electrode voltage is thus lowered, operating said writing gun for bombarding said phosphor layer with said beam of electrons for erasing information from a portion of said phosphor layer toward which said beam of high-velocity electrons is directed.
 8. The method according to claim 7 wherein said voltage on said collector electrode is lowered by more than the voltage difference between written and unwritten areas of said phosphor layer.
 9. The method according to claim 7 wherein the electron beam from said writing gun during erasure is deflected over an area, wherein erasure is desired, in a systematic raster sequence.
 10. The method according to claim 7 wherein said collector electrode voltage is rapidly reduced for effectively cutting off operation of said flood guns without erasure of said charge image by said flood guns.
 11. The method of operating a bistable storage tube, said tube including a phosphor layer on an insulative support member, a target electrode on said support member beneath said phosphor layer, a collector electrode supported in contact with said phosphor layer on the opposite side of said phosphor layer from said target electrode for collecting secondary electrons, a writing electron gun for providing a beam of high-velocity electrons, and holding means for bombarding the phosphor layer with low-velocity electrons, said method comprising: writing a charge image on selected areas of said phosphor layer by bombarding said phosphor layer with said beam of high-velocity electrons from said writing gun in said selected areas, with said low-velocity electrons from said holding means producing continued secondary emission from said selected areas to cause bistable storage of such image, discontinuing emission of said low-velocity electrons from said holding means, altering the normal relative voltage differential on said target and collector electrodes for increasing the voltage difference between the written areas of said charge image and the potential of said collector electrode, with the collector electrode being relatively negative compared with written areas of said phosphor layer, said difference being greater than the voltage difference between written and unwritten areas of said phosphor layer, bombarding a selected area of said phosphor layer to be erased with said beam of high-velocity electrons from said writing gun while deflecting said beam of electrons in a systematic pattern to charge written areas, toward which said beam is directed, downward toward the potential of said collector electrode, and restoring the normal voltage differential between said target and collector electrodes as well as restoring operation of said holding means for producing secondary electron emission from written areas not erased for retention of information represented thereby.
 12. The method according to claim 11 wherein said selected area of said phosphor layer to be erased is bombarded with said beam of high-velocity electrons for a period long enough to lower such selected area to a level which resides below the first crossover voltage of the secondary electron emission characteristic of said phosphor layer after said normal voltage differential is restored.
 13. The method according to claim 11 including deflecting said beam of electrons in said systematic pattern with a reduced beam current prior to bombarding said selected area of said phosphor layer to be erased, and adjusting the size and position of said systematic pattern while viewing the same upon said phosphor layer prior to erasure for selecting the area to be erased.
 14. The method according to claim 13 wherein said systematic pattern comprises a raster sequence.
 15. The method according to claim 11 wherein the increased voltage difference between written areas of the target and the potential of the collector electrode is at least approximately twice the voltage difference between written and unwritten areas of said phosphor layer. 